Polycarbonates are well known as tough highly impact resistant polymers. However as an amorphous material, above a certain critical thickness the polycarbonate loses its impact strength and becomes rather weak and brittle. This critical thickness lies between 1/8 inch and 1/4 inch. Molded parts of 1/8 inch thickness are highly resistant to impact in a notched test while molded parts of 1/4 inch thickness are above the critical thickness and demonstrate poor notch impact results as well as brittleness. Additionally polycarbonates are quite sensitive to certain environmental stresses such as organic solvents.
Many blends of polycarbonate with other polymers have been developed in an attempt to improve these properties in polycarbonate. Some of these blends have met with significant degree of success. However these admixtures can create new problems. In order for an admixture to be successful, the composition must be compatible. The admixture components should mix and maintain the blend integrity upon molding. This is evidenced by little or no delamination in the molded part and an impact resistant, ductile knit line when a molded part is formed in a mold having two ports of entry for the thermoplastic admixture (double gate test system).
A new polycarbonate composition has been discovered which combines substantially improved impact resistance in thick section with improved organic solvent resistance while demonstrating good compatibility.